I posted recently about my fuel pressure dropping when parked. I think this is a different problem, it's been going on a long time (years) and just wanted to ask if anyone else has run into this. My IAC system seems good, it's clean, and when I start up cold it idles at 1400 then drops to 900 in a minute or two. Problem happens only under very specific conditions. On a warm restart, like after say 20 minutes, it will fire right up, then taper down and stall. Restart it and it fires right up and idles fine at 900.

If the A/C was left on it will do this 100% of starts. If A/C was turned off, it does this maybe 1/2 the time. Engine doesn't seem to miss, it just tapers off and dies, then restarts and idles fine. It's like on the first warm restart, the IAC isn't getting a signal to set the idle speed. I can live with it, and have for several years, in fact if I fixed it I'd probably be lost without the annoyance. In recent service/maintenance work I've run the voltage and resistance checks on the IAC solenoid, EGR position sensor, cleaned the IAC valve, replaced the O2 sensor and the FPR, and replaced the high pressure fuel pump and inline filter. I have a new EGR position sensor ordered because mine had a flat spot in the resistance test under vacuum.

I've recently pulled codes several times using an Innova code reader, but each time I pull codes they are wildly different. The only consistent code I always get is a 31. After I replace the EGR position sensor I'll check them again but I don't expect much from this code reader.

Just curious if anyone else has this problem with an EEC IV 302 motor.

On the bright side, my crystal ball sees a fresh 302, carbureted, headers, and no smog stuff in the near future

 

Code 31 will trigger other errant Codes due to the fact that the EEC-IV computer is trying to compensate for the EVP sensor's erratic information coming back to it. Which Codes are manifest depends on a host of variables from humidity and ambient air temperature to the actual coolant and engine temp etc.

Your description of it winding down and dying is an interesting issue in that the "test" to troubleshoot for a functioning IAC is to disconnect it with the engine already warmed up and idling. A properly functioning IAC will cause the exact kind of behavior you described IF it the engine is at normal op. temp. AND the IAC loses power. The fact that you can actually MAKE this happen with the introduction of the A/C compressor circuit would indicate to me that either the IAC itself is sticking closed periodically or that there is a questionable connection somewhere between the IAC and the computer.

The IAC operates on an on-off frequency generated by the EEC computer wherein the solenoid opens and closes very quickly in rapid succession. This generates friction and heat to some degree within the IAC itself AND the wiring to it. The operational frequency is increased while the compressor/clutch circuit is engaged to afford the slight boost in RPM's at idle needed to compensate for the additional load on the engine. If I had to guess (and that is all one could do without actually taking an oscilloscope to the IAC signal wiring and/or a thermal reading of the wiring itself). I would posit that ether some connection in the wiring between the IAC and the computer is weak enough that the thermal variance and/or the variance in frequency (translate that as vibration) mucks with the signal coming into the IAC on the next startup and it clears as soon as the connection is restored.

Do you get ANY Codes that indicate IAC troubles or even a "lean" condition?

 

At different times I've gotten codes 31, 32, 41, 42, 45, and some others that I can't recall. Dumb me, I tossed the sheet that had code results from 3 or 4 recent tests, when I saw the results were so erratic. The 41 code (engine running) indicates o2 sensor voltage low/system lean in my book, that's the only one that referred to a lean condition. Only appeared once. Since I already had a new o2 sensor ready to install I went ahead and did that. Truck runs good, never stalls in stop and go, just that odd restart stall. All the components that I've unplugged, I've cleaned the plug contacts with electrical cleaner.

I bought this Bronco with 55,000 miles on it and now it has 156,000. Been doing this pretty much since I bought it. My new EGR position sensor should be in this week. Once that's installed, I'm thinking I'll clear the memory codes, reset the PCM, drive her a day or two, then run one more code test. Thanks for the great explanation of how the IAC works.

 

I replaced the faulty EGR position sensor yesterday. Reset the computer and cleared the continuous codes. Took her out for a 20 mile drive, highway and stop-go driving. Came home and with engine all warmed up, pulled codes.

Got these: 31 (engine off) 11 ( continuous) 44, 31 (running).

This morning I re-checked the codes and got 11 (engine off) 51 (continuous), 44 (engine running). So, my 31 codes went away but now I'm getting a thermactor fault and an ECT fault. I went ahead and ran the supply voltage and resistance checks on the ECT. Ohms checked out right on with cold engine and warmed up engine. Supply voltage consistent at 4.8.

Trying to chase codes that keep changing is enough to make a young man old, and an old man cranky. The Bronco is running good, still stalling on warm restart with A/C on. I'm going fishing.

 

What about the vacuum lines? are they complete and connected?. Changing could actually point at a faulty ECM, do a visual inspection, see if there are no blown capacitors.

 

All the vac lines are original stock, not tampered with, and I can't detect any vac leak noises or symptoms. What's the sign of blown capacitors? Burn marks inside the cover?

Any pics or info on where you got your roof tent?

 

You can get a similar product from ARB or other Australian brands, mine is made in Venezuela, but you get the idea, here are a couple of pics for you:






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That's a neat setup. My thanks to you, Greystreak, Kemicalburns and all the other folks on the forum who share their knowledge of these old Fords so we can keep ours chugging down the highway.